German patent 33 25 854 discloses a motor vehicle braking system the brake pressure of which is controlled by a brake pressure booster in response to deceleration of the vehicle achieved by braking. This system is intended to make it easier for the driver to meter the brake pressure as he pushes down the brake pedal and, in particular, a certain brake pedal actuation always is to give him the same braking effect no matter how his vehicle is loaded. To accomplish that, the vehicle deceleration is measured and compared with the force exerted on the pedal in conventional manner. When the pedal force applied by the driver brings about a retardation of the vehicle which is less than a given rated value, the brake pressure booster is actuated automatically so that the desired value of vehicle deceleration will be achieved.
In the case of DE 33 25 854 the difference in pressure inside the brake pressure booster is controlled exclusively by a control valve selectively connecting one of the two chambers of the brake pressure booster to a pressure source. If the electrical system should fail (rupture of a cable or the like) the brake pressure booster does not work.
In British patent application 89 18 159, which is not a prior publication, a motor vehicle braking system is specified which has a brake pressure booster integrated in the antilock and in the antislip controls. That brake pressure booster is modified so as not to include a customary mechanical valve for pressurizing the high pressure chamber but instead only an electromagnetic valve system which is controlled by a central processor in response to certain measured values. The values relied upon for measurement especially are the rotational speeds of the wheels (vehicle deceleration), the pedal force, etc. For instance, if a rotational speed sensor for the wheels indicates that they are spinning without the brake pedal being actuated then the control system, acting through operation of the electromagnetic valve, causes the brake pressure booster to become active so as to automatically retard the spinning wheel. Under other circumstances, too, the brake pressure booster is controlled automatically by way of the electromagnetic valve system. For example, the brake pressures generated by the brake pressure booster in response to the force applied on the pedal are measured and "smoothed" in the sense that the most gentle braking will be obtained, without any abrupt onset or termination of the effect of the brake pressure booster. If there is a failure in the electrical system the brake pressure booster fails also with this motor vehicle braking system. This shortcoming is true also of the state of the art according to DE 33 25 854 and DE 22 09 091.
It is known from U.S. Pat. No. 4,667,471 to connect an electromagnetic valve in series with a conventional pedal-operated booster valve. For normal application of the brakes, in this case the control connection between the vacuum chamber and the booster chamber is established by the pedal-operated valve which, however, is backed up by a downstream electromagnet. The brake pressure booster is controlled automatically by the electromagnetic valve so as to facilitate starting up a hill.
U.S. Pat. No. 4,759,255 discloses a low pressure brake pressure booster with which control may be effected either through a pedal-operated mechanical valve or through an electromagnetic valve. In that case, too, the electromagnetic valve is actuated in order to make it easier to start driving the vehicle uphill.